At the present time, virtually all printed copy is produced through the use of three basic types of printing plates. One type is a relief plate which prints from a raised surface. Another type is an intaglio plate which prints from a depressed surface. The third type is a lithographic plate which prints from a substantially fiat surface which is neither appreciably raised above nor appreciably depressed below the adjacent and surrounding non-printing areas. Printing is occasioned by an ink's respective affinity and/or aversion to areas of different chemical properties. Lithographic printing plates are commonly processed to have water-repellent (hydrophobic), oil-receptive (oleophilic) image areas and water-receptive (hydrophilic) non-image areas.
Prior to processing for use, conventional lithographic plates will typically have a hydrophobic, photoreactive polymeric layer (i.e. photoresist) coated or otherwise deposited atop a hydrophilic substrate.
In preparing a conventional lithographic plate for use on a printing press, the plate is first exposed to actinic radiation. Specific chemical reactions are caused to occur in the plate's photoresist by exposure to actinic radiation. Such photoinduced chemical reactions may either reduce or enhance the solubility of the photoresist, depending on whether the resist is negative-working or positive-working. In negative-working plates, exposure to actinic radiation will generally cause a "hardening" of the photoresist. In positive-working plates, exposure to actinic radiation will generally cause a softening or solubilization of the photoresist.
After photoexposure, a wet development step is normally conducted. The objective of such wet development is to remove those areas of the photoresist which have undergone photoinduced chemical change (in positive plates) or those which have not been photoexposed (in negative plates). Solvation under conventional development techniques will typically involve treating the exposed plate with developer in a developing bath. For negative-working resists, the developer will swell and dissolve the unexposed portions of the resist. The developer should not swell the exposed portions or distortion of the developed image may result. For positive-working resists, the response of the unexposed and exposed coatings are reversed, but the same general principles apply.
As a result of the preferential solvation and washing away of portions of the photoresist, corresponding portions of the underlying hydrophilic substrate are uncovered. For negative-working plates, the aforementioned hydrophobic image areas correspond to the portions of the photoresist remaining after solvation and washing. The aforementioned hydrophilic non-image areas correspond to uncovered portions of the substrate. The image and non-image areas thus differentiated, the processed plate may then be mounted onto a printing press and run.
Encumbered by required wet development, the processing of conventional lithographic plates prior to their use on a printing press is both time and labor consuming and involves considerable use of organic chemicals. It will be appreciated that there is a considerable desire for means that would satisfactorily eliminate or reduce conventional lithography's long-felt dependency upon the conduct of wet development and thereby permit use of lithographic plates on a printing press immediately after exposure without required post-exposure prepress processing.
In the past, dry developable lithographic printing plates have been suggested which enable the wet processing steps of lithographic printing plates after exposure to be omitted and printing to be conducted by directly mounting the exposed plates on a printing press. Among printing plates that may be characterized as on-press developable (or related thereto) are: e.g., U.S. Pat. No. 4,273,851, issued to Muzyczko et al. on Jun. 16, 1981; U.S. Pat. No. 4,879,201, issued to Hasegawa on Nov. 7, 1989; U.S. Pat. No. 4,916,041, issued to Hasegawa et al. on Apr. 10, 1990; U.S. Pat. No. 4,999,273, issued to Hasegawa on Mar. 12, 1991; and U.S. Pat. No. 5,258,263, issued to Z. K. Cheema, A. C. Giudice, E. L. Langlais, and C. F. St. Jacques on Nov. 2, 1993.
Despite the methodologies and approaches embodied in the aforementioned patents, there is a continuing need for a lithographic printing plate that can be readily developed on a printing press and that produces a plate having durable image areas needed for good run length. Applications for such on-press developable printing plates have been filed.
U.S. patent application Ser. Nos. 08/147,045 and 08/146,711, filed by W. C. Schwarzel, F. R. Kearney, M. J. Fitzgerald, and R. C. Liang on Nov. 1, 1993, describe a photoreactive polymeric binder that may be used to enhance photospeed in either conventional plates or on-press developable lithographic printing plates. Briefly, a polymer of m-isopropenyl-.alpha.,.alpha.-dimethylbenzyl isocyanate is derivatized for vinyl group reactivity by reacting the isocyanate groups thereof with a hydroxyalkyl acrylate, such as 4-hydroxybutyl acrylate. The resulting photopolymeric binder provides higher photospeed than compositions containing non-reactive binders typically utilized in the production of printing plates. Lithographic printing plates utilizing the photoreactive polymeric binder have good durability (as manifested by good run-length) and can be developed using relatively weak developers. As to the preparation of the photoreactive binders, the applications describe a method of copolymerizing m-isopropenyl-.alpha.,.alpha.-dimethylbenzyl isocyanate through complexation with an electron-deficient monomer (e.g., maleic anhydride) to accelerate free radical copolymerization with other monomers. The maleic anhydride accelerated process is kinetically more efficient and provides greater monomer-to-polymer conversion. Use of the resulting product in the photoresist of a lithographic printing plate improves its adhesion. The disclosures of commonly assigned U.S. patent application Ser. Nos. 08/147,045 and 08/146,711 are hereby incorporated by reference. Reference is also made to U.S. Pat. App. Att'y Dkt. No. C8024, commonly assigned and filed on Apr. 27, 1995.
U.S. patent application Ser. No. 08/147,044, filed by F. R. Kearney, J. M. Hardin, M. J. Fitzgerald, and R. C. Liang on Nov. 1, 1993, describes the use of plasticizers, surfactants and lithium salts as development aids for negative-working, on-press developable lithographic printing plates. Briefly, plasticizers, which are dispersible or soluble in press fountain solutions and soluble in acrylic monomers and oligomers, are incorporated into a photoresist. Such plasticizers make the photoresist more permeable to fountain solution prior to crosslinking, while being easily extracted with ink and fountain solution after crosslinking. The surfactants facilitate the dispersion of hydrophobic imaging compositions in the fountain solution and reduce scumming. Further, lithium salts may also be incorporated into the photoresist to disrupt hydrogen bonding of, for example, urethane acrylate polymers which tend to associate by hydrogen bonding, thus enhancing developability. The disclosure of commonly assigned U.S. patent application Ser. No. 08/147,044 is hereby incorporated by reference.
U.S. patent application Ser. No. 08/146,479, filed by L. C. Wan, A. C. Giudice, W. C. Schwarzel, C. M. Cheng, and R. C. Liang on Nov. 1, 1993, describes the use of rubbers and surfactants to enhance the durability of on-press developable printing plates. The rubbers are preferably incorporated into a photoresist as discrete rubber particles. To ensure a uniform and stable dispersion, the rubber components are suspended in the photoresist preferably by means of surfactants having HLBs approximately between 7.0 and 18.0. The disclosure of commonly assigned U.S. patent application Ser. No. 08/146,479, is hereby incorporated by reference.
It will be appreciated that underlying each of the referenced applications is an acknowledgment of the difficulty in the realization of both "on-press developability" on the one hand and "durability" on the other. Such difficulty is believed to originate from an apparent contradiction between photoresist resist removability ("developability") on the one hand and "durability" on the other: To make a photoresist more durable was to make a photoresist less developable.
To illustrate the point, it will be noted that conventional wet development techniques are oftentimes based upon the use of strong solvents. Accordingly, tough and comparatively resilient resists can be more liberally prepared and utilized. The dictates of more narrowly defined parameters for "on press developability", govern latitude in the choice of photoresist components, in general, and accordingly--as evident in the above-referenced applications--result in an unconventional resist configuration.
While the several resist configurations encompassed by the referenced applications provide good on-press developability, it is noted that accomplishment of optimal resolution and photospeed appeared frustrated in certain plates exposed to prolonged and elevated temperatures, such extreme conditions potentially arising during unmitigated shipping and/or storage.
Determination of the cause of such loss of photospeed and resolution is made difficult by the complex interrelationships of the several systems incorporated into such photoresists. Regardless, it is emphasized that the photoresists at issue are based on flee-radical initiated photopolymerization, the particular embodiments specifically employing 2-phenyl-4,6-bis(trichloromethyl)-1,3,5-triazine. In view of experimentation, in view of several and possible alternative factors, and in view of the unconventional and a generally low Tg configuration of the on-press developable photoresists, it is believed that loss of resolution and photospeed is attributable to the volatilization of 2-phenyl-4,6-bis(trichloromethyl)-1,3,5-triazine, the photooxidant being key to the free radical cure systems of the photoresist.
Such volatilization may be prevented by increasing the binder content of the photoresist, for example, thereby increasing Tg and the "rigidity" of the photoresist. However, a more "rigid" resist would be correspondingly more difficult to develop "on press". Rate of development and in turn resolution in part depends on the diffusion rate of developer (cf., fountain and ink solution) into the photoresist; a more "rigid" image coat may decrease the diffusion rate.
Alternatively, the prevention of volatilization of certain triazines can be accomplished by providing a ballast group thereon. Such methodology is employed, for example, in U.S. Pat. Nos. 5,298,361 (issued to Bonham et al. on Mar. 29, 1994); U.S. Pat. No. 5,064,741 (issued to Koike et al. on Nov. 12, 1991); U.S. Pat. No. 4,837,128 (issued to Kawamura et al. on Jun. 6, 1989); and U.S. Pat. No. 5,340,697 (issued to Yoshimoto et al. on Aug. 23, 1994). However--in employing such strategy to the particularly preferred triazine used in the above-referenced applications--unacceptable "scumming" defects were observed. (See Example 2, infra).
In light of the above, there is a need to develop a strategy for preventing the volatilization of a 2-phenyl-4,6-bis(trichloromethyl)-1,3,5-triazine from a photocurable composition, the strategy not leading to the aforementioned "scumming" defects when the photocurable composition is employed as a photoresist in a lithographic printing plate. The strategy should also desirably be directed toward reducing light absorption of the triazine at about .lambda.&gt;360 nm, even when the triazine forms a charge transfer complex with a leuco triarylmethane dye.